In packaging operations, transparent thermoplastic films are frequently used which are relatively transparent to radiant energy. Accordingly, it is generally required to heat these films by convection. Typically, this is accomplished with a flow of hot air.
Modern skin-packaging machines are designed for use in many applications other than retail display packages. For example, they may be used in protecting products for shipment or storage. In such applications, the machines may be used intermittently and sporadically. Because of this sporadic or cyclic use, skin packaging and other machines normally use electricity as a source of energy for heat.
The same considerations apply to vacuum forming machines which utilize essentially the same equipment as skin packaging machines except that vacuum forming machines produce an article.
Skin packaging and vacuum forming devices generally require heating the heat-softenable film for a precise length of time prior to starting the machine cycle, and maintenance of that heat for a precise length of time during the cycle, after which the heat should be quickly and dramatically stopped. Continuously heated elements, if properly designed, provide a well-dispersed pattern of controlled heat. However, in order to quickly and accurately control the heating time before and during the machine cycle, the heat source must either be intermittently blocked with gates, doors or movable louvers, or more commonly the entire oven must be moved over and away from the film area. This requires additional mechanical devices and uses a large amount of floor space to accommodate the travel of the oven. This type of heat source is also very wasteful of energy since it requires a considerable amount of time to reach operating temperature, and it must be kept heated, even when no packages or formed pieces are being produced.
Quartz heaters have also been used in thermoforming and skin packaging devices. In this type of heater, a resistance wire or ribbon is wound around the outside, or the inside, of a quartz tube. When electric current is applied, the resistance element heats up fairly rapidly (about four seconds) causing the tube to emit heat rather quickly, and when the current is switched off, the heater cools down. There are several shortcomings in the quartz type heater approach. The quartz tubes are very fragile, causing great difficulties in shipping because of breakage. Even more important, the fragile nature of the product severely limits the length of a heater, making the method impractical for larger machines. Finally, with the heat generally concentrated in the tube areas there is a strong tendency for uneven heating of the plastic. Further, the quartz tube acts as a heat sink which requires time to cool when the electric current to the heater is terminated.